In conventional television systems, the video signal is composed of a succession of lines. In the NTSC system, each field comprises 262.5 lines, and each two consecutive fields are interlaced. Thus, in one frame, comprising two fields, the entire raster of 525 lines is scanned.
Because of the relationship between subcarrier frequency and line frequency in the NTSC system, the phase relationship between subcarrier and horizontal sync repeats every four fields. If there is no motion between fields, fields 3 and 4 are identical to fields 1 and 2, except that the phase of reference subcarrier relative to horizontal sync is reversed. Therefore, a signal conforming to the standards prescribed for field 3 or 4 can be generated from a signal representing field 1 or 2 by reversing the phase of reference subcarrier, and so it is possible to generate a four field color sequence from one odd-numbered field and one even-numbered field.
In a conventional television graphics system, a composite video signal representing a picture that is to be modified in some manner is decoded into component form using a frequency-selective filter or a vertical or horizontal comb filter, and the component signals are converted to digital form and written into a frame buffer. The components may be three primary color components (R, G, B) or a luminance component and two color difference components. Because two consecutive fields of the NTSC signal can be recreated from the immediately preceding two consecutive fields, the frame buffer of the conventional television graphics system stores only two fields of the video signal. The stored data is modified in some manner to reflect the desired modification of the picture, and the modified data is read out of the frame buffer and encoded in order to recreate a four field color sequence.
Decoding of a composite video signal using a frequency-selective filter generates spurious information, because high frequency color information may be interpreted as luminance information and luminance information within the chrominance frequency band may be interpreted as color information. In a comb filter, the luminance and chrominance information are separated by forming the sum and difference of elements of the composite video signal representing different locations in the picture, e.g. two different sections of a line in the case of a horizontal comb filter, and decoding using a comb filter generates spurious information unless the color content of the picture at those different locations is precisely the same. Consequently, use of a conventional television graphics system tends to degrade the video signal.
In a conventional, component-based, television graphics system, each line is resolved into about 760 pixels in order to enable spatial frequencies of up to about 380 cycles/line to be accurately displayed. Although relatively high spatial frequencies are necessary for displaying brightness or luminance information, the human eye is much less sensitive to high frequency color information than it is to high frequency brightness information. Therefore, a substantial amount of the data stored in the frame buffer of a conventional color component (e.g. R, G, B) television graphics system does not contribute to improving the picture that is produced using the video signal generated by the graphics system. Also, because operation of the conventional television graphics system takes place in the component domain, the operator of the system is shielded from some of the limitations that arise when the signal generated by the system is encoded into composite form and is displayed using a conventional television receiver. For example, cross-color and cross-luminance effects are not observed when a component video signal is used to drive a component monitor, and therefore the operator of the system may be less sensitive to these effects than an operator using a monitor that receives encoded video signals.
In recent years it has become common to use a frame synchronizer in order to synchronize composite video signals received from different sources, so as to enable switching between sources without undue degradation of the output video signal from the switcher. A frame synchronizer includes a digital memory into which one of the video signals is written, and it is read out in synchronism with the other video signal. Frame synchronizers operate in the composite domain, typically sampling the video signal at four times subcarrier frequency (910 samples/line in the case of the NTSC system). However, frame synchronizers are pure delay devices, and do not provide any means for purposefully modifying the picture represented by the video signal.
A television production switcher may have associated with it a digital video effects (DVE) system that includes a frame store. The DVE system enables one or more pictures, represented by one or more video signals, to be modified, e.g. by keying a foreground scene of one picture over a background scene of another picture. However, the DVE system operates in the component domain and it is not able to perform synthetic graphics operations.